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Metal roofing -how to choose

Various roof failures, including asphalt shingle blow-off, and premature curling and cracking, has led homeowners to look for more reliable and durable solutions. As such, people often start asking about metal roofing.

Metal roofing continues to be more and more popular, despite its initial cost, because people are getting tired of having roofs that fail much before their warranties expire. The metal product offerings come in a dizzying variety of materials, appearances, textures, colours, and performance. How does one wade through all the sales claims and marketing hype to decide which options are truly the best for you? It helps to remember that there is no product invented by humans that is truly perfect in every circumstance. So the exercise is to find the product whose weaknesses are minimized in your particular circumstances, and whose strong points are fully delivered. It should not be a surprise that proper preparation and installation play a very big role in the final quality of the result. So let’s examine a selection of products and discuss how their weak (and strong) points should be considered in your selection process.

Metal Roof Benefits

Metal roofs offer many benefits to homeowners, including:

much better life-time value compared to asphalt shingles

better wind resistance (metal systems are usually capable of resisting winds of 120 mph (192 km/hr), and more if high-wind installation methods are used.

better environmental impact (fewer non-renewable resources consumed over the life of the roof),

longer-lasting curbside appeal

Metal Roof Materials

Metal roof coverings usually are made from aluminum or steel, transformed into shingles or panels, that can be bare, painted, or covered with granules. Specialty roofs can also be made from copper sheet, zinc, stainless steel, and untreated iron. In this post, we will deal with aluminum and steel as the two most common metals.

Aluminum

Aluminum is used to manufacture both shingles and panels. The aluminum alloy used is usually 3003 or 3105. The minimum thickness allowed under the building code is 0.49mm (0.019″). The aluminum is almost always painted, using weather-resistant paint systems like Kynar or less expensive ones like siliconized polyester, where the paint is “baked on”.

Aluminum Shingle Roof, painted brown

Strong points of aluminum

In no particular order, the strong points are listed below (note that the list is not exhaustive):

The material is light, typically weighing about 0.5lb/sq.ft. (2.5 kg/sq.m.) for the thickness of 0.49mm (0.019″). This compares to about 1 lb./sq.ft. for steel, 2-3lbs/sq.ft. for asphalt shingles, 6 lb/sq.ft. for fibro cement, and upwards of 9 lb/sq.ft. for clay tile, slate, or similar products.

The material is easy to bend, making it quite suitable for DIY application where specialized tools are often not available.

Aluminum does not rust. The oxidation of aluminum results in aluminum oxide, which is a very hard, transparent mineral which then “skins” the aluminum and prevents further oxidation.

Aluminum has a very low heat capacity, so it absorbs and retains very little heat energy.

Aluminum is very reflective in the infrared, which means that most infrared energy is reflected away, not absorbed as with most non-metal materials.

Weak points of aluminum

Again, in no particular order:

Being a soft metal, aluminum is easily dented or damaged by snow movement, hail, walking on the roof, or falling branches.

Being a soft metal, underlying deck surface imperfections telegraph themselves through the shingle or panel surface.

While aluminum does not rust, is is a very reactive metal, and can corrode galvanically if in contact with a non-aluminum metal such as steel or copper.

Aluminum has a relatively low melting point of around 644C (1190F), which may be an issue if there is a strong fire.

Aluminum has a coefficient of expansion that is roughly twice that of steel, which needs to be taken into account if long sheets are to be used.

Painted aluminum can be very slippery when wet, allowing snow avalanches off aluminum roofs in winter, and being dangerous for walking on in other seasons.

Unless the pretreatment of aluminum is well done, the adhesion of primer and paint to aluminum can be a problem.

Aluminum roofing products

Aluminum roofing products usually come in two forms: shingles and panels. The shingles can be made to resemble 3-tab asphalt shingles, or slate, or wood shake and shingles. These are made by manufacturers in many countries, including Canada, USA, China, Australia, France, etc.

Depending on the manufacturer, the shingles usually feature an interlocking design and have a pattern pressed or embossed onto the surface. Some are quite flat, others have rather deep profiles. Claims about durability range from 50 years to 100 years. The installation methods vary by manufacturer, but they share the need to use an exclusively-aluminum set of flashings, nails, and accessories to avoid galvanic corrosion.

The panels can be relatively large rectangular sheets stamped with a clay tile or similar design or can be formed as relatively narrow but long vertical panels with side seams. As with the shingles, the claims for durability range from 50 years to 100 years.

Aluminum Installation

All metal roofing products depend critically on the installation quality to achieve their objectives of performance and longevity. Each roof edge or joint has a specific flashing (or even a system of flashings) to anchor the product and ensure its ability to channel water and resist wind uplift or damage. Since products designed for steep roof installation are generally NOT waterproof (because they are designed to shed water), these products rely on the system of flashings, membranes, underlayments, and waterproofing techniques to keep the water out. If these are not well executed, then you will have a beautiful, leaky roof.

Aluminum roofs have to be installed with aluminum flashings and aluminum fasteners. Any contact with non-aluminum metal has to have an insulating layer between that metal and aluminum.

Failures associated with aluminum roofing

Almost always, the failures are associated with the failure of the installer to take into account the way the metal will act on a given roof.

If they install the product over existing roofing, then there is an excellent chance that the imperfections of the roof underneath the product will “telegraph” themselves though the product as the weight of snow pushed the material down.

Snow avalanches occur on smooth-surfaced metal roofs in winter, and depending on a number of factors (amount of snow, slope, amount of heat loss from the attic, etc.) can have enough mass to damage (and tear off) gutters, or damage anything the snow mass falls on (car windshields, pumps, shrubbery, decorative objects, tables, even cast-iron rails). This becomes particularly problematic if the snow mass lands on your neighbour’s property.

Snow avalanches can also be dangerous to pets and people who can be walking or playing below when the avalanche happens.

Movement of snow and ice along valleys will sometimes catch and tear the shingles alongside the valleys.

Caulking on aluminum will break more readily due to the stresses related to expansion and contraction of aluminum flashings, so it is more important to ensure that the waterproofing detailing does NOT rely on caulking.

Walking on a wet roof is extremely dangerous, as the wet surface is very slippery. This is not unique to aluminum – it will be true for any smooth, painted metal surface. However, it needs to be taken into account if access to the roof (as an emergency exit, for instance) is required.

Best Practices (with aluminum)

Ensure that the style and design of the roof allow the snow to slide off in a safe manner and to fall in places where such snowfall is not dangerous. Therefore, roofs that tend to slope to the front and back (where you have driveways, walkways, porches, etc.) are usually not the best candidates for these types of roofs, unless proper precautions (snow fences, snow guards or deflectors) and included in the planning of the roof.

Ensure that the slope of the roof is adequate to allow the water and snow to be shed off the roof. This usually means a slope of 4:12 or more (4:12 refers to the ratio of the vertical rise the horizontal run).

Ensure that the roof covering is put over solid decking with appropriate waterproofing. The practice of many contractors to install metal roofing over existing roofing (ie, without stripping the old roof off down to the roof deck), is a poor practice in my opinion.

Every roof edge (eave, rake, endwall/sidewall, transition, ridge/hip) has to be terminated with an appropriate flashing and overlapped in the appropriate manner.

All components of the roof system must be made from compatible metal (ie, aluminum) to avoid galvanic corrosion.

Any contact with dissimilar metal must be avoided, or if it cannot be avoided, then isolated to minimize the possibility of galvanic corrosion.

Contact with various preservatives (as in treated wood), or oxidation products from metals (rust, etc.), must be avoided as these compounds can and do react with aluminum, causing corrosion or discoloration.

Given the amount of expansion and contraction aluminum can undergo, both the positioning and fastening of shingles or panels must be carefully considered, to allow for enough overlap and movement.

Steel

Steel is the most common metal roofing material and has been used in various forms for well over 100 years. The primary weakness of steel has been its tendency to rust, which was combatted either with the use of elements that would prevent steel from rusting (known as stainless steel), or with coatings that protected the steel (steel with zinc, or galvalume, or other coatings). Steel plated with zinc is known as galvanized steel, and the degree of protection is determined by the thickness of the zinc plated onto the steel sheet. Roofing products made from steel range in thickness from very thin (29 gauge – .014″ or 0.36mm), to very substantial (20 gauge – 0.0375″ or 0.95mm), and depending on the product are available as shingles, panels, or sheets, in various profiles, and finishes.

Granulated Steel Shingles (Allmet – Granite Ridge)

Example of screw-through steel panel roof

Hidden-fastener standing-seam steel roof on heritage home

Steel panels with Terra-Cotta style.

Strong points of steel

In no particular order, the strong points are listed below (note that the list is not exhaustive):

Steel products are generally very strong, resistant to deformation and damage.

Steel is very ductile which means it can be formed into a wide variety of shapes through stamping, drawing, bending, etc.

The technologies used to minimize or prevent rusting are well known and are very effective.

The steel products used for residential installation weigh less than the equivalent surface area of materials such as asphalt shingles, clay tile, slate or fibro-cement products (usually about 1 lb/sq.ft. or 4.9 kg/sq.m., compared to about 2-3 lbs/sq.ft. for asphalt shingles, 6-9 lbs/sq.ft. for tile, slate or fibro-cement).

Steel products are available in a wide variety of finishes ranging from “bare” mill finish to painted, to coated with additional materials such as mineral granules. This results in a wide choice of potential looks and appearances.

Weak points of steel:

Again, in no particular order:

Iron or steel used in the majority of residential roofing products can rust and requires appropriate surface treatment to reduce or stop the rusting.

Steel is both resistant to deformation and to being cut – so usually special tools and techniques are required to carry out the finishing trimming or bending on site.

When working with steel, care must be taken to avoid damaging the protective coating(s) that prevent rusting.

Each manufacturer has a system of installation designed for each type of product, and it is very important for the installer to follow the installation methods created for that particular system. The system of installation includes the various components (including flashings, fasteners), the roof preparation, the waterproofing techniques to be used, and the methods of dealing with roof penetrations. The system of installation differs considerably from the techniques used for asphalt roofing, so it is important that the installation team is well-trained in the particular system used.

Depending on the product, manufacturer, and chosen finish, manufacturer warranties range anywhere from limited warranties of 20-30 years, to all-inclusive warranties of 50 years or more. Keeping in mind that warranties are primarily marketing documents, it pays to read the fine print of exactly what the warranties are promising and under what conditions will they consider claims. For instance, it is common to have a 50-year non-prorated transferable warranty, with the fine print indicating that the paint is only warranted for 10-20 years (and that only if you can decipher the technical language used). Furthermore, most systems will not be warranted unless the installation was done by a suitably-trained installer.

With the newest coatings and treatments, I can expect that most products will outlive their manufacturer warranties and that if properly installed, the roofs on which they are placed will be essentially maintenance-free for at least 50 years.

Steel Installation

As noted earlier, each roofing product/system has a particular set of components, techniques and installation methods that must be followed to achieve the proper end result. However, in comparison to aluminum, steel can be difficult to work with and generally requires specialized tools for forming, bending, or cutting. These tools tend to be expensive and represent an investment by the contractor or installer in carrying out such installations. As such, except for the screw-through steel panels, most steel product installations should be left to trained and well-equipped professionals.

Depending on the system, the products can be installed directly onto the roofing deck or may require strapping or battens. The choice of whether to use strapping or battens depends partly on the product, but also on the other aspects of the roof such as the presence of older roofing materials, need for ventilation or requirements to level the deck. For example, metal roofing can have condensation form on the underside of the metal panels, and it is important to anticipate this reality and prepare the roof for dealing with this, either by installing appropriate waterproofing and underlayments or providing for additional ventilation, or both.

While the majority of steel roof manufacturers allow installation of their products over existing roofing, I consider this to be a poor practice for the many reasons I’ve already listed in other posts. When things go wrong, it’s much more complicated to fix a roof with a layering of roofing materials, rather than dealing with the relatively simple issue of repairing a single roof covering.

Failures associated with steel roofing

As with aluminum, the failures are associated with the failure of the installer to take into account the way the metal will act on a given roof, and the failure of the installer to use the proper installation methods and techniques.

When installing over existing roofing, failure to provide for more than one line of defense against water infiltration. Once the caulking is breached, the water is free to enter the roof system.

Failure to anticipate snow movement and avalanches when installing smooth-surfaced products. This generally means damage to gutters and whatever the snow landed on.

Premature rusting of product due to improper handling during installation, or due to improper methods of making cuts in the metal. Metal panels can be heavy, and the installers must provide for adequate manpower to move the panels onto the roof without dragging them along the ground or scratching the surface finish. As well, using tools such as grinders to cut metal is expressly forbidden by virtually all the metal manufacturers, but used by contractors in a hurry.

Failure to provide for proper drainage paths for water in the flashing and waterproofing system. In general, flashings have multiple roles, and a very important one is the channeling of water out of the roof system to the outside. If the proper detail work is not done in layering such flashings, the water will end up being directed INTO the roof system, instead of being brought out. Combined with the presence of old roofing, this creates a situation where the water will accumulate and contribute to deck failure through rot, and contribute to the growth of mold.

Mixing of dissimilar metals on the same roof system, leading to galvanic corrosion.

Failure of non-steel components such as foam strips, waterproofing seals, etc., when exposed to the sun. This is particularly notable in the cheaper roof systems that rely on neoprene washers, foam sealing strips, and the like to prevent water entry. Failure of such systems can happen in as little as 10 years.

Bleaching, fading or peeling of paint. This is particularly an issue of cheap paint (and paint application) associated with the cheap screw-through panels. The paint used for the more expensive panels tends to be of the type that resists UV damage for a much longer time (Kynar or equivalent).

Damage to the steel panels due to snow avalanche from a higher level. This can occur with the thinner panels and ones where the installation method suspends the metal away from the deck surface. It is important for the installer to consider the snow movement in order to decide which gauge of metal, and type of installation to use.

Wet painted metal can be extremely slippery (just as with aluminum), and this MUST be taken into account if access to the roof surface is needed (for emergency exit or for maintenance reasons).

Condensation damage (including rot and mold) when the condensation issues were not adequately planned for.

Best Practices (with steel)

If considering a smooth-surfaced metal product, ensure that the style and design of the roof allow the snow to slide off in a safe manner and to fall in places where such snowfall is not dangerous. Therefore, roofs that tend to slope to the front and back (where you have driveways, walkways, porches, etc.) are usually not the best candidates for these types of roofs, unless proper precautions (snow fences, snow guards or deflectors) and included in the planning of the roof.

Ensure that the slope of the roof is adequate to allow the water and snow to be shed off the roof. For most products, this usually means a slope of 4:12 or more (4:12 refers to the ratio of the vertical rise the horizontal run). Certain products such as hidden-fastener standing seam panels can be installed on very low slopes (1:12 or even less) IF appropriate preparation and installation is followed.

Ensure that the roof covering is put over solid decking with appropriate waterproofing. The practice of many contractors to install metal roofing over existing roofing (ie, without stripping the old roof off down to the roof deck), is a poor practice in my opinion. If strapping or battens are needed, consider the underside condensation potential and whether additional preparation is needed to handle that condensation.

Every roof edge (eave, rake, endwall/sidewall, transition, ridge/hip) has to be terminated with an appropriate flashing and overlapped in the appropriate manner. This is particularly important in systems that have built-in gutter channels, which can be expected to fill up with water.

All components of the roof system must be made from compatible metal (ie, steel or iron) to avoid galvanic corrosion.

Any contact with dissimilar metal must be avoided, or if it cannot be avoided, then isolated to minimize the possibility of galvanic corrosion.

Contact with various preservatives (as in treated wood), or oxidation products from metals (rust, etc.), must be avoided as these compounds can and do react with steel, causing corrosion or discoloration.

Work with the metal using the appropriate, specialized tools. They have been designed to make the job easier and to minimize the possibility of damage to the materials.

For every junction, provide for adequate waterproofing since metal joints are never waterproof for a long time, even with caulking.

Summary:

Metal roofs give the best long-term value for most homeowners, provided that they are properly chosen and properly installed. They also have to be considered as part of the overall roof system, which includes everything from the ceiling gyprock outwards to the roof covering (ie, vapour and air barrier, insulation, ventilation, deck design and structure, water-channelling, and integration with the rest of the housing envelope). I have a metal roof system on my own house (18 years old now) and will install an appropriate metal system for every relative and friend who asks me because they deliver the best value, performance, maintainability and curb-side appeal. However, we have also had to remove and/or repair many metal roof systems because the salesmen and homeowners didn’t consider the many aspects of such roof systems that a good, experienced installer will (or should) take into account. There are many companies that sell metal roofs. There are only a few that do it really well.

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Owner of PRS Roofing Inc., Permanent Roofing Systems, and PRS Consulting. Occasional inventor. Preacher and teacher, with the favorite topic being how to do it right the first time. Technical consultant to many DIY roofing installers.